Communication networks use “timing advance”, also denoted as “TA”, to adjust the uplink transmission timing of individual wireless terminals. These TA adjustments ensure that the uplink, UL, transmissions from different terminals are aligned in time at the involved network receivers. Aligning the different UL transmissions in time at the network receiver(s) preserves orthogonality in the UL direction.
In the context of wireless communication networks based on Long Term Evolution, LTE, standards as promulgated by the Third Generation Partnership Project or 3GGP, the wireless terminals are referred to as user equipments or UEs, and the involved radio network nodes are a type of base station referred as an “evolved NodeB” or “eNB”. The transmit timing of the UEs under the control of the same eNB should be adjusted to ensure that the UL signals transmitted by them arrive at the eNB with the same time alignment.
More specifically, the UL signals from the different UEs should arrive well within the cyclic prefix or CP. The “normal” CP length is about 4.7 μs. This alignment ensures that the eNB receiver is able to use the same resources (i.e. same Discrete Fourier Transform, DFT, or Fast Fourier Transform, FFT resource) to receive and process the signals from multiple UEs.
The eNB maintains the required UL timing alignment of the UEs under its control by sending TA commands to the individual UEs. In turn, the TA commands generated for a particular UE depend on measurements made by the eNB with respect to UL transmissions received from the UE. For example, the eNB measures two-way propagation delay or round trip time, RTT, for a particular UE to determine the TA value required for the UE. Here, the TA value or command represents a negative offset at the UE between the start of a received downlink subframe and a transmitted UL subframe. By varying the particular offset used by a particular UE, the eNB compensates for the different propagation delays between it and respective ones of the UEs under its control, and thus keeps all of the UEs synchronized to the same downlink/uplink frame/subframe timing used on the air interface.
For a TA command received by a given UE on subframe n, the corresponding adjustment of the UL transmission timing shall by applied by the UE from the beginning of subframe n+6. The TA command indicates the change in UL timing relative to the current UL timing of the UE transmission as multiples of 16 Ts, where Ts=32.5 ns and is referred to as the basic time unit in LTE.
In the case of a random access response, an 11-bit TA command, TA, for a timing advance group or “TAG” indicates NTA values by index values of TA=0, 1, 2, . . . , 1282, where an amount of the time alignment for the TAG is given by NTA=TA×16, and where NTA is defined below. In other cases, a 6-bit TA command, TA, for a TAG indicates adjustment of the current NTA value, NTA,old, to the new NTA value, NTA,new, by index values of TA=0, 1, 2, . . . , 63, where NTA,new=NTA,old+(TA−31)×16. Here, adjustment of the NTA value by a positive or a negative amount indicates advancing or delaying the UL transmission timing for the TAG by a given amount, respectively.
Timing advance updates are signaled by an eNB to a UE in Medium Access Control, MAC, Protocol Data Units, PDUs.
In another aspect of wireless terminal operations, a given UE or other wireless terminal typically performs inter-frequency and inter-RAT, Radio Access Technology, measurements during so called “measurement gaps”, unless the UE is capable of performing measurements without gaps. A measurement gap in this context is a time period during which the UE is not scheduled for reception and/or transmission by the network and where, correspondingly, the UE can use its receiver circuitry to make signal measurements on other frequencies and/or with respect to other RATs.
For UEs that require measurement gaps to make and report inter-frequency and inter-RAT measurements, the network node must determine the particular measurement gap configuration to be used by the UE. The LTE standards define two periodic measurement gap patterns, each having a measurement gap length of 6 ms. The first pattern, denoted as pattern #0, has a repetition period 40 ms, whereas the second pattern, denoted as pattern #1, has a repetition period of 80 ms. The measurements performed by the UE are then reported to the network, which uses them for various tasks.
LTE provides the following possible measurements by a UE during its configured measurement gaps: (a) inter-frequency cell detection or cell identification; (b) inter-frequency RSRP measurement, where RSRP denotes Reference Signal Received Power; (c) inter-frequency RSRQ measurement, where RSRQ denotes Reference Signal Received Quality; (d) inter-frequency RSTD, where RSTD denotes Reference Signal Time Difference; (e) inter-RAT cell identification, e.g., identification of any one or more of GSM/GERAN, WCDMA, UTRA TDD, and CDMA2000 networks; and (f) various inter-RAT measurements, such as Common Pilot Channel, CPICH, Received Signal Code Power, RSCP, CPICH Carrier-to-Noise ratio, Ec/No, and GSM carrier Received Signal Strength Indicator or RSSI.
The measurement gaps are used in all duplex modes of operation, which include Frequency Duplex Division or FDD, Time Division Duplex or TDD, and Half-Duplex FDD or HD-FDD. In HD-FDD, the UL and downlink, DL, transmissions take place on different paired carrier frequencies but not simultaneously in time in the same cell. The use of HD-FDD therefore means that the UL and DL transmissions take place in different time resources, e.g., different symbols, time slots, subframes or frames. In other words, the UL and DL subframes do not overlap in time. The number and location of subframes used for DL, UL or unused subframes can vary on the basis of frame or multiple of frames.
The air interface in LTE networks is referred to as E-UTRA, denoting Evolved UMTS Terrestrial Radio Access, where UMTS denotes Universal Mobile Telecommunications System. TDD operation in E-UTRA specifies measurement gaps with particular subframe offsets, which are best understood with reference to FIG. 1, depicting a “Type 2” frame structure for TDD operation, based on a 5 ms “switch point” periodicity between DL and UL subframes. Correspondingly, FIG. 2 illustrates the defined UL/DL configurations defined in LTE for TDD operation.
In the table of FIG. 2, “D” denotes a DL subframe, “U” denotes an UL subframe, and “S” denotes a “Special” subframe that includes both DL and UL portions. The structure of these special subframes is seen in FIG. 1, where “DwPTS” denotes the DL portion of the subframe, “UpPTS” denotes the UL portion of the subframe, and “GP” denotes a Guard Portion or Guard Period between the DL and UL portions.
FIG. 3 illustrates, that for UL/DL Configuration #0, measurement gaps with offsets of 3 and 8 subframes relative to the frame border are squeezed in between two uplink subframes. FIG. 4 illustrates, that for UL/DL Configurations #0, #1 and #6, measurement gaps with offsets of 2 and 7 subframes are squeezed in between a special subframe and an UL subframe.
These observations are notable with respect to the assumptions underlying the defined measurement gap configurations and their associated timings. Namely, one of the assumptions made when defining existing UE behavior for measurement gaps was that the measurement gap was to be defined with respect to the DL timing. That is, measurement gaps were to be aligned with DL subframes. Moreover it was assumed that transmissions overlapping the measurement gap were to be dropped.
In particular, the 3GPP technical specification TS 36.133 V10.14.0, section 8.1.2, defines the following UE (E-UTRAN corresponds to LTE) behavior: in the uplink subframe occurring immediately after the measurement gap, the E-UTRAN FDD UE shall not transmit any data, and the E-UTRAN TDD UE shall not transmit any data if the subframe occurring immediately before the measurement gap is a downlink subframe. This second provision covers LTE TDD operation, but does not cover the case when the measurement gap is positioned between two uplink subframes, or between a special subframe and an uplink subframe.
This omission might be justified if considering only the autonomous change of timing, where the UE is allowed to autonomously change its transmit timing by at most 17.5×Ts (0.57 μs) per 200 ms, where Ts is the basic unit of time in LTE, provided that it is not the first transmission after Discontinuous Reception (DRX). The relative position of the gap would differ because it is defined from UL timing instead of DL timing, but the length would be 6 ms, as required.
In a practical implementation at some point in time the UE has to plan for switching the radio receiver from intra-frequency to inter-frequency, and later back again. Additionally, the UE may need to plan for when to carry out automatic gain control, when to start recording In-phase/Quadrature, I/Q, samples for offline processing, and/or configuring hardware accelerators for online processing, and/or configuring software for control and processing.
Suppose that this planning is done, say, less than 200 ms in advance. In that case, the autonomous change of timing would potentially result in that measurement gap timing at the UE would move at most ±0.6 μs, for measurement gaps that are positioned between uplink activities. This could be compensated for by removing 0.6 μs from the beginning and the end of the measurement gap, as a margin for the change in gap position. The impact would be negligible.
When taking TA commands into account, however, the situation becomes somewhat more problematic. TA adjustments have no impact on the measurement gaps that are covered by the discussion above, because their positions are determined by the DL timing. However, commanded changes in UL timing via the TA mechanism may have a large impact on the measurement gaps whose positions are determined by the UL timing of the UE.
Although not very likely, it is possible that a UE receives one TA command every DL or special subframe, with each such TA command to be applied by the UE four subframes later. Each such TA command may change the UL timing within the range −31×16Ts to 32×16Ts (about ±17 μs). If one assumes that the aforementioned planning is done 20 ms in advance, it would mean that the maximum timing change would be about ±180 μs for UL/DL Configuration #1. How much of this that actually can be applied depends on the size of the guard period in the involved special subframe configuration and on the aggregated timing advance at the time when the planning is carried out, because the aggregated timing advance is bounded.
If the uncertainty in measurement gap positioning here is handled in the same manner as described above for autonomous UL timing changes made by the UE, the worst-case reduction of the measurement gap would be about 0.36 ms. This reduction value reflects the fact that the UE has to plan for the maximum of the aggregated TA change in either direction, 20 ms in advance. Reducing or shrinking the measurement gap by that amount would compensate for the uncertainty but it would also leave too little radio time for the gap to be useful for cell search and measurements.
FIG. 5 illustrates minimum guaranteed measurement gaps for the following scenarios: (A) FDD single component carrier, 3GPP TS Rel.8 and onwards; (B) TDD single component carrier, 3GPP TS Rel.8 and onwards; (C) FDD CA, 3GPP TS Rel.10 and onwards; (D) TDD CA with same UL/DL allocation on both carriers, 3GPP TS Rel.10 and onwards for single TAG, and 3GPP TS Rel.11 and onwards for multiple TAGs—the case where the Ems long measurement gap is positioned between two UL subframes is considered; and (E) TDD CA with different UL/DL allocation on the carriers, 3GPP TS Rel.11 and onwards—the requirement of being able to aggregate carriers with different UL/DL configurations is still under discussion, and the case when the Ems long measurement gap is positioned between two UL subframes is considered.
In particular, in the context of FIG. 5, TA commands received during the 6 subframes before a measurement gap will modify the length of the measurement gap. The figure uses shaded subframes to depict those subframes where no serving cell transmission or reception is to be carried out under existing rules. FIG. 6 illustrates several known mitigations for guaranteeing a minimum measurement gap length of 6 ms, both with and without the involved UE having received a TA command before the gap.
It has also been suggested that a UE may skip transmissions in the UL subframe following immediately after a measurement gap, should the gap be preceded by an UL subframe or a special subframe, where such subframe formats are defined in 3GPP TS 36.211. In particular, for example special subframe definitions, see 3GPP TS 36.211 V10.7.0, section 4.2. This approach, however, is in tension with the preexisting assumption by eNBs that a given UE may always be scheduled immediately after a measurement gap.
Other factors to consider include the fact that it is unlikely that a UE will receive four consecutive maximum TA adjustments within a measurement gap period. Further, note that a DL subframe immediately following a measurement gap can be used for data reception in any case. Of further note is whether the subframe occurring immediately before a measurement gap is an UL subframe. In general, whether or not a UE, e.g., the particular UE implementation may dictate whether or not a UE operating according to an E-UTRAN TDD configuration, can transmit data in an UL subframe immediately following a measurement gap.
In terms of the types of signal measurements that can be reported, FIG. 7 illustrates a known Channel Quality Indicator, CQI, table, which shows the mapping defined between CQI values or indexes, and corresponding Modulation and Coding Schemes, MCSs. One sees that higher-order modulations and more efficient encodings can be used with higher CQIs. A given UE may be configured to periodically report CQI to its supporting network. Thus, a scheduler associated with or included in a supporting eNB may use CQI reports for link adaptation.
For LTE TDD, the reporting period can be: 1, 5, 10, 20, 40, 80, and 160 ms, respectively, and with some restrictions on the UL/DL configuration in use. See, e.g., 3GPP TS 36.213 V10.12.0, section 7.2.2. A typical network configuration uses a reporting period in the range of 5-40 ms. The reporting may also be aperiodic by which the UE receives an indication in Downlink Control Information or DCI that it shall send a CQI report to the eNodeB.